The present invention relates to a hydraulically assisted steering system for a motor vehicle.
Such steering systems are known from practical use. They are described, for example, in the reference book xe2x80x9cChassis Engineering: Steering Systems and Power-Assisted Steering Systemsxe2x80x9d, Vogel Buch-Verlag, Wurzburg (1992). On page 130 a recirculating ball steering system with an external vane-cell pump, a steering valve and a steering gear case is illustrated, wherein the steering gear case contains a working piston which acts upon the steering shaft. A conventional power steering system of said design comprises, in addition to the rotary slide valve or rotary piston valve, one or two valves for limiting the lock of the steering shaft as well as a pressure limiting valve and a secondary suction valve, wherein the pressure limiting valve limits the pressure in the flow line of the power steering system in an upward direction and the secondary suction valve, in the event of failure of the servo assistance, enables manual displacement of the hydraulic fluid from one working chamber through a bypass into the second working chamber. The limiting valves for the angle of the steering shaft are disposed in the steering gear case or in the working piston itself and operated directly by cams associated with the steering shaft or by striking against the housing base. Upon attainment of a specific steering angle, said stop valves or limiting valves establish a hydraulic connection between the two working chambers of the working piston. The servo assistance is therefore reduced and the steering angle is in said manner effectively limited.
The described steering system therefore requires the installation of altogether up to four purely mechanical hydraulic valves, which have to be designed in accordance with the special requirements and fitted into the steering system. In practice this entails relatively high costs because each individual valve has to be separately manufactured, tested and fitted in a fluid-tight manner into the steering system.
The object of the present invention is therefore to guarantee the described functions with a simpler valve arrangement. A further object of the present invention is to provide a valve arrangement which enables different modes of operation in dependence upon the driving conditions of the motor vehicle.
Said object is achieved in the case of recirculating ball steering by a steering system having the features of claim 1 and in the case of rack-and-pinion steering by a steering system having the features of claim 2.
Because there is associated with the hydraulic system an electromechanical valve which in the open state hydraulically connects the working chambers, the mode of operation of the power steering may be influenced in dependence upon the measurement results of electrical sensors or upon other control commands. In particular it is possible to use a sensor, which monitors the wheel angle of the steered wheels or alternatively the steering wheel position, in order from a selectable wheel lock angle on to reduce or interrupt the servo assistance through connection of the two working chambers, with the result that the function of the steering angle limiting valve is taken over.
In said case, it is advantageous when an electronic circuit is provided for triggering the valve and for controlling the pressure difference in the working chambers. By said means the opening and closing of the valve may be handled flexibly and optionally even the extent of the servo assistance may be adjusted. It is further advantageous when at least one sensor for the position of the steering is provided, wherein the circuit is arranged so as to trigger the valve in dependence upon the sensor. It is thereby possible first to simulate the already mentioned steering stop through reduction of the servo assistance. It is however also possible through purposeful influencing of the servo assistance in dependence upon the steering angle to achieve a specific characteristic curve of the servo assistance. When the valve is opened given positions of the steering system which correspond to a maximum desired wheel angle, turning of the steering beyond said maximum wheel angle and consequent damage to the vehicle is reliably avoided.
When a sensor for the speed of the motor vehicle is associated with the circuit and the maximum desired wheel angel is variable in dependence upon the speed, at higher speeds it is possible to avoid a steering stop which would lead to unsafe driving conditions. At low speeds, e.g., when maneuvering, on the other hand, a wheel angle of a magnitude not desirable under normal conditions may be permitted.
A particularly compact design is achieved when the valve is disposed inside the steering gear case. The valve may, on the other hand, be mounted at a particularly accessible point without modifications to existing steering gear cases when it is disposed outside of the case, particularly in the region of a hydraulic line. Here, for example, it may be provided that the flow line of the hydraulic pump and the storage container are connected to one another, with the result that the two working chambers of the power steering system are indirectly hydraulically coupled. The ensuing effect is substantially the same.
It is further advantageous when a pressure sensor is associated with the circuit and the valve is opened upon attainment of a specific pressure or of a specific pressure difference. In said manner it is possible to dispense with the mechanical pressure limiting valve. The valve may also provide an autonomous mechanical pressure limitation in that it opens without triggering by the circuit given a specific pressurization. Especially given cold hydraulic fluid, this may prevent an excess pressure in the hydraulic system without the electronic circuit having to operate for said purpose. When the valve is opened in the event of a pressure drop in the hydraulic system, then, given discontinuation of the servo assistance already, the purely mechanical operation of the power steering by manual force is not impaired because a bypass is created from one working chamber to the other working chamber. The same applies to the situation where the valve is open in the de-energized state. With said embodiment, it is possible to dispense with the secondary suction valve which is otherwise provided for said situation.
Finally, it is advantageous when the valve in the open state presents a throttle cross section which, given pressurization by the hydraulic system, ensures a minimum pressure difference between the working chambers. The effect achieved thereby is that in the event of failure of the pressure limiting valve, e.g. as a result of parting of a cable, while travelling the steerability of the vehicle with a minimum of servo assistance is guaranteed.
There now follows a description of the present invention with reference to two embodiments for recirculating ball steering systems. The same applies to rack-and-pinion steering systems, to which the embodiments are transferable without any difficulty.